Wafer processing films

ABSTRACT

The present invention relates to a wafer processing film used for the prevention of breakage upon grinding a semiconductor wafer. The wafer processing film has a base film an adhesive layer provided on one surface of the base film and a synthetic resin film arranged on the adhesive layer. The surface roughness of the surface of the synthetic resin film in contact with the adhesive layer is not greater than 2 μm. 
     The present invention provides a wafer processing film which can protect the surface of a wafer from contamination and corrosion after grinding the wafer.

DESCRIPTION

1. Technical Field

The present invention relates to a wafer processing film which is usedwhen grinding wafers such as silicon wafers so as to prevent theirbreakage.

2. Background Art

Semiconductor integrated circuits (ICs) are generally fabricated byslicing a semiconductor material such as a high-purity silicon singlecrystal into a wafer, forming integrated circuits therein throughetching or the like, and then dicing the resultant wafer into chips.

In a step in which the back side of an IC wafer is ground and polished,it is a routine practice to affix a wafer processing film, which has anadhesive layer, onto the surface of the wafer so that breakage to thewafer can be prevented and the grinding work can be facilitated.

A wafer processing film is generally produced by coating an adhesive onone surface of a base film and then drying the same to form an adhesivelayer on the base film. A synthetic resin film called a "separator" isapplied over the adhesive layer to protect the adhesive layer duringstorage or transportation of the wafer processing film, whereby thewafer processing film has a structure such that the adhesive layer issandwiched between the base film and the separator. Upon grinding thewafer, the separator is peeled off and the wafer processing film isadhered to the wafer. After completion of the grinding, the waferprocessing film is peeled off from the wafer.

If air is included between the adhesive layer of the wafer processingfilm and a wafer when the processing film is applied to the surface ofthe wafer, the adhesive tends to remain on the surface of the wafer whenthe film is peeled off from the wafer. This residue causes corrosion ofthe resulting IC wafer so that the performance of the IC is lowered.

Further, adhesion of any foreign matter to the surface of the wafer fromthe film also induces corrosion of the resulting IC wafer, resulting ina reduction in the performance of the IC.

To cope with the problem of air inclusion, the following method isproposed in Japanese Utility Model Laid-Open No. 131631/1983 by way ofexample. According to the method, a resin film whose surface roughnesshas been improved to the range of ±0.1 μm by coating a silicone-baserelease agent is employed as a separator (release layer), so that thesurface of a pressure-sensitive adhesive layer as an adhesive layer isprevented from being roughened by the separator and, upon application ofthe wafer processing film to a wafer, air is prevented from beingincluded between the adhesive layer of the film and the wafer. Becauseof the use of the silicone-base release agent, this method is, however,accompanied by the drawback that the release agent is transferred to theadhesive layer to contaminate the surface of the semiconductor wafer.This method is therefore not satisfactory.

According to the method proposed in Japanese Patent ApplicationLaid-Open NO. 177423/1988, a semiconductor wafer fixing member with arelease layer made of low-density polyethylene or polymethylpentene isused, and as a result, contamination of the semiconductor wafer due totransfer of the release agent or the like from the separator can beavoided. However, because of the high surface tension of low-densitypolyethylene, the peelability between a pressure-sensitive adhesivelayer as an adhesive layer and the release layer is so poor that thesurface of the adhesive layer is roughened when the release layer ispeeled off from the semiconductor wafer fixing member. As a result, anumber of fine air-filled spacings is formed at the interface betweenthe adhesive layer and the wafer. When the wafer processing film ispeeled off after the back side of the wafer has been ground andpolished, the adhesive tends to remain around the air-contacted spots onthe surface of the wafer and the adhesive residue causes corrosion ofthe wafer.

As has been described above, it is the current circumstance that, as awafer processing film to be employed upon grinding an IC wafer, there isno useful film capable of preventing contamination and corrosion of thesurface of the wafer.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to overcome the above-describedproblems and to provide an excellent wafer processing film, which isuseful upon grinding a wafer. Specifically, it is an object of thepresent invention to provide a wafer processing film which can leave thesurface of a wafer free from contamination and corrosion.

The present invention therefore provides a wafer processing filmcomprising a base film, an adhesive layer provided on one surface ofsaid base film and a synthetic resin film arranged on said adhesivelayer. The surface roughness of the surface of said synthetic resinfilm, said surface being in contact with said adhesive layer, is notgreater than 2 μm. The synthetic resin film will hereinafter be calledthe "separator".

Corrosion of the surface of the wafer occurs if, upon peeling off waferprocessing film from the surface of a wafer subsequent to grinding ofthe wafer, any adhesive remains on the surface of the wafer or thesurface of the wafer is contaminated with any material adhered thereto.

The adhesive tends to be left behind on the wafer surface especiallyaround air-contacted spots thereof when air is included between theadhesive layer and the wafer. To avoid the inclusion of air between theadhesive layer and the wafer, it is desirable to make the surface of theadhesive layer flat, said surface to be adhered to the surface of thewafer. For this purpose, it is effective to reduce the surface roughnessof the separator.

On the other hand, to avoid the adhesion of any contaminant to thesurface of a wafer, it is effective not to coat the surface of theseparator with a wafer contaminant such as a release agent.

It is also desired to use a separator having good releasability, becausepoor releasability results in roughening of the surface of the adhesivelayer when the separator is peeled off from the adhesive layer.

It is also effective to avoid the use of a lubricant or the like in alarge amount in a separator since the inclusion of the lubricant or thelike in a large amount causes bleeding of the lubricant or the like andresults in its transfer to the surface of the adhesive layer.

With the foregoing in view, the present inventors have proceeded with aninvestigation resulting in the present invention.

Owing to the use of a separator having little surface roughness, theseparator does not cause roughening of the surface of the adhesive layerin the wafer processing film according to the present invention. As aresult, no air is included between the adhesive layer and the wafer whenthe wafer processing film is applied to the wafer. The adhesive istherefore not left behind on the surface of the wafer when the waferprocessing film is peeled off from the wafer. Accordingly, the use ofthe wafer processing film according to the present invention has theexcellent advantage that a wafer can be processed without contaminationand corrosion.

BEST MODE FOR CARRYING OUT THE INVENTION

Wafers to which wafer processing films of the invention can be appliedinclude not only silicon wafers but also any semiconductor wafers suchas germanium wafers, gallium-arsenic wafers, gallium-phosphorus wafersand gallium-arsenic-aluminum wafers.

No particular limitation is imposed on the production process ofseparators to be used in the invention. They can be made of a syntheticresin film produced by any conventional process such as extrusion, blownfilm extrusion or calendering.

Exemplary materials for separators include synthetic resins such aspolyolefins, polyesters, polyamides, polyacrylic resins andpolypropylene. Among these, polypropylene is preferred. Use of asynthetic resin having high surface tension and poor releasability as aseparator is not preferred because the separator roughens the surface ofan adhesive layer.

The use of a biaxially-stretched polypropylene film as a separator ispreferred. When an unstretched film or uniaxially-stretched film is usedas a separator, the unstretched film may be elongated and theuniaxially-stretched film may be torn in the stretched direction whenthe separator is peeled off from the wafer processing film Theworkability is hence impaired whichever problem takes place. Thetransparency of a film is generally improved when subjected to biaxialstretching. Therefore, biaxially-stretched films are also preferred fromthe standpoint of prevention and control of wafer contamination becausethe inclusion of foreign matters can be easily discovered.

The thickness of a separator is preferably 10-2,000 μm, with 20-200 μmbeing more preferred. If it is smaller than 10 μm, the workability isimpaired. If it is greater than 2,000 μm, the flexibility is reduced tothe point that the workability is impaired. Thickness outside the aboverange is therefore not preferred.

The surface roughness of the separator on the side to be brought into acontiguous relation with the adhesive layer is not greater than 2 μm interms of the maximum height over the standard length of 0.8 mm asstipulated in JIS B 0601. The term "standard length" as used herein,means a standard measurement length of a sample when the surfaceroughness of a surface of the sample is measured. If this surfaceroughness exceeds 2 μm, the separator roughens the surface of theadhesive layer. As a result, when the separator is peeled off and thewafer processing film is adhered to a wafer, air is included between thewafer and the adhesive layer. When the wafer processing film is peeledoff from the wafer, the adhesive remains in the form of spots on thesurface of the wafer. Such adhesive residue causes corrosion of thewafer. Such large surface roughness is therefore not preferred.

Preferred as a separator to be used in the invention is a syntheticresin film having a Shore "D" hardness of 30 or higher as measured inaccordance with ASTM D-2440. If this hardness is lower than 30, the filmhas insufficient stiffness and the workability is impaired.

The separator employed in the invention preferably has a lubricantcontent not greater than 0.5 part by weight per 100 parts by weight ofthe synthetic resin. More preferably, the lubricant content is 0-0.1part by weight Lubricant content greater than 0.5 part by weight resultin bleeding of the lubricant on the surfaces of the separators so thatthe associated adhesive layers are contaminated. Such high lubricantcontent is therefore not preferred. Lubricant content not greater than0.1 part by weight is substantially free from lubricant bleeding,whereby the associated adhesive layers are free from contamination. Itis most preferable that no lubricant is contained.

Exemplary lubricants usable in the invention include those generallycalled "lubricants", e.g., glycerin monoesters such as glycerinmonostearate, bisamides such as methylene bisstearylamide and aliphaticamides such as erucic amide; as well as those added as hydrochloric acidscavengers, such as calcium stearate.

The total content of additives other than lubricants, namely,stabilizers (antioxidants) such as 2,6-di-tert.-butyl-4-methylohenol andinorganic fillers such as silica and talc should preferably be nothigher than 1 part by weight per 100 parts by weight of the resin inorder to prevent contamination of the adhesive layer.

The synthetic resin film for the separator can be suitably chosen fromcommercial products. From the standpoint of prevention of wafercontamination, a film produced in an environment whose air cleanlinessclass is Class 100,000 or less is preferred. The term "Class 100,000 orless" indicates that not more than 100,000 dust particles of 0.5 μm orgreater are contained per one cubic feet of the air in the workingenvironment.

No particular limitation is imposed on the production method or the kindof base film employed in the invention. It can be suitably selected fromsynthetic resin films produced by conventional processes such asextrusion, blown film extrusion or calendering. Examples of the materialof the base film include thermoplastic elastomers such as ethylene-vinylacetate copolymers, polybutadiene, plasticized vinyl chloride resin,polyolefins, polyesters and polyamides; and synthetic rubbers such asdiene rubbers, nitrile rubbers, silicone rubbers and acrylic rubbers.Among these, films having a Shore hardness of 40 or lower as measured inaccordance with ASTM D-2440 are preferred in order to effectively absorbimpacts and protect the wafer during grinding.

The thickness of the base film can be suitably determined depending onconditions such as the shape of a wafer to be protected, the surfacecondition of the wafer and the manner of grinding. In general, 20-2000μm is preferred.

No particular limitation is imposed on the type of the adhesive employedin the adhesive layer of the invention as long as it has adhesiveness. Aconventional adhesive such as an acrylic adhesive or a rubber-baseadhesive can be used.

The thickness of the adhesive layer formed on the surface of the basefilm can be suitably determined depending on conditions such as thesurface conditions of a wafer, the shape of the wafer and the manner ofgrinding. Generally, 2-100 μm is preferred, with 5-50 μm being morepreferred.

As a method for coating the adhesive to the base film, a conventionalcoating method, for example, roll coating, gravure coating, bar coatingor the like can be employed. One side of the base film is coatedentirely.

The present invention will hereinafter be described by the followingexamples although they do not limit the invention.

The evaluations in the following examples and comparative examples wereeffected by the following methods:

a. Degree of Oxidation

Generally, the surface of even a fresh silicon wafer has been lightlyoxidized and includes silicon oxide. When this wafer surface issubjected to an elementary analysis by ESCA, the ratio of the peak areaof oxygen to that of silicon is 1.30 on average. When the wafer surfaceis corroded, the amount of silicon oxide in the surface increases sothat the ratio of the peak area of oxygen to that of silicon becomesgreater than 1.30. The degree of oxidation was judged on the basis ofthe ratio of 1.30.

Measuring Apparatus and Conditions for ESCA

Apparatus: "ESCA LAB MK II", trade name Manufactured by VG ScientificCo.

X-ray source: Mg Kα rays

X-ray output: 300W

Vacuum level for measurement: 1×10⁻⁹ mbar max.

b. Degree of Contamination

The degree of contamination due to any remaining adhesive and/or thelike was investigated using a laser surface inspection apparatus,"HLD-300B" (trade name) manufactured by Hitachi Denshi Engineering, Ltd.The degree of contamination was expressed in terms of the number ofcontaminated spots having a diameter of at least 0.2 μm.

c. Breakdown Voltage

Voltage was gradually increased across the terminals of each IC wafer.The voltage at which the IC was broken was recorded as its breakdownvoltage. The measuring method comprised sticking a wafer processing filmon the front side of an IC wafer, grinding and polishing the back sideof the wafer, peeling off the film, allowing the wafer to stand for 2000hours in an environment of 70° C. and 80% R.H., and then measuring thebreakdown voltage. The breakdown voltage of the IC was 22 V when thewafer was free from corrosion. This voltage was employed as a standardvalue. The breakdown voltage drops as the corrosion of a waferincreases.

EXAMPLE 1

A commercial ethylene-vinyl acetate copolymer resin film of 200 μm thickwas provided as a base film. One side of the base film was subjected tocorona treatment. An acrylic adhesive (product of Mitsui ToatsuChemicals, Inc.; "AROMATEX", trade mark) was coated on thecorona-treated side of the film by a roll coater and was then dried,whereby a wafer processing film provided with an adhesive layer of about30 μm thick was prepared.

To 100 parts by weight of polypropylene resin, 0.1 part by weight oferucic amide was added as a lubricant. In an environment of Class100,000, a biaxially-stretched polypropylene film having a surfaceroughness of 1 μm max. was produced as a separator.

The separator was arranged over the adhesive layer of the adhesive film,so that a wafer processing film was obtained.

The separator was peeled off from the wafer processing film and wasadhered to a surface of a mirror wafer (diameter: 4 inches). After theywere left for 24 hours, the film was peeled off from the wafer and thedegree of oxidation and contamination of the wafer were evaluated. Theresults are shown in Table 1.

In another case, the separator was peeled off from the wafer processingfilm and the film was adhered to the surface of an IC silicon wafer(diameter of 4 inches) with aluminum patterns formed thereon. Afterbeing left for 24 hours, the film was peeled off and the breakdownvoltage of the IC silicon wafer was measured. The results are shown inTable 1. No problems were encountered in workability when the separatorwas peeled off from the wafer processing film.

EXAMPLE 2

An experiment was conducted in a similar manner to Example 1 except thatthe amount of erucic amide was changed to 0.5 part by weight and themaximum value of the surface roughness of the separator was controlledat 2 μm. The results are shown in Table 1. No problems were encounteredin workability when the separator was peeled off from the waferprocessing film.

EXAMPLE 3

An experiment was conducted in a similar manner to Example 1 except thatthe amount of erucic amide was changed to 0.5 part by weight and auniaxially-stretched polypropylene film was produced as a separator. Theresults are shown in Table 1. Tearing of the separator in the stretcheddirection was observed on extremely rare occasions when the separatorwas peeled off from the wafer processing film.

EXAMPLE 4

An experiment was conducted in a similar manner to Example 1 except thata commercial butadiene rubber sheet 300 μm thick was used as a base filmand a biaxially-stretched polypropylene film with an addition of 10 partby weight of erucic amide and having surface roughness of 2 μm max. wasemployed as a separator. The results are shown in Table 1. No problemswere encountered in workability when the separator was peeled off fromthe wafer processing film.

EXAMPLE 5

An experiment was conducted in a similar manner to Example 1 except thata commercial butadiene rubber sheet 300 μm thick was used as a base filmand a biaxially-stretched polyethylene terephthalate film with anaddition of 0.1 part by weight of erucic amide and having surfaceroughness controlled at 1 μm max. was employed as a separator. Theresults are shown in Table 1. The releasability of the separator was notvery good.

EXAMPLE 6

An experiment was conducted in a similar manner to Example 1 except forthe use of a biaxially-stretched polypropylene film which was obtainedwithout adding erucic amide and with a surface roughness of 0.5 μm max.The results are shown in Table 1. No problems were encountered inworkability when the separator was peeled off from the wafer processingfilm.

Comparative Example 1

An experiment was conducted in a similar manner to Example 1 except thata biaxially-stretched polypropylene film obtained by adding 2.5 parts byweight of erucic amide and having a surface roughness of 3 μm max. wasused as a separator. The results are shown in Table 1. No problems wereencountered in workability when the separator was peeled off from thewafer processing film.

Comparative Example 2

An experiment was conducted in a similar manner to Example 1 except thata commercial butadiene rubber sheet of 300 μm thick was used as a basefilm and a biaxially-stretched polyethylene terephthalate film with anaddition of 0.5 part by weight of erucic amide and having a surfaceroughness of 5 μm max. was employed as a separator. The results areshown in Table 1. The separator exhibited poor releasability.

Comparative Example 3

An experiment was conducted in a similar manner to Example I except thata commercial butadiene rubber sheet of 300 μm thick was used as a basefilm and an unstretched low-density polyethylene film with an additionof 1.0 part by weight of erucic amide and having a surface roughness of3 μm max. was used as a separator. The results are shown in Table 1. Theseparator exhibited poor releasability and, when peeled off, theseparator was elongated and had poor workability.

Comparative Example 4

An experiment was conducted in a similar manner to Example 1 except thata biaxially-stretched polypropylene film obtained by adding 0.5 part byweight of erucic amide and having a surface roughness of 3 μm max. wasused as a separator. The results are shown in Table 1. No problems wereencountered in workability when the separator was peeled off from thewafer machining film.

                                      TABLE 1                                     __________________________________________________________________________    Separator                Evaluation results                                   Surface    Lubricant     Degree      Breakdown                                roughness  content       of   Degree of                                                                            voltage                                  (μm)    (wt. part)                                                                          Material*.sup.1                                                                       oxidation                                                                          contamination                                                                        (V)                                      __________________________________________________________________________    Ex. 1                                                                              1     0.1   Biaxial PP                                                                            1.30  15    22                                       Ex. 2                                                                              2     0.5   Biaxial PP                                                                            1.30  26    22                                       Ex. 3                                                                              1     0.5   Uniaxial PP                                                                           1.31  44    22                                       Ex. 4                                                                              2     1.0   Biaxial PP                                                                            1.31  78    22                                       Ex. 5                                                                              1     0.1   Biaxial PET                                                                           1.32 116    22                                       Ex. 6                                                                                0.5 0     Biaxial PP                                                                            1.30  9     22                                       Comp.                                                                              3     2.0   Biaxial PP                                                                            1.58 875    16                                       Ex. 1                                                                         Comp.                                                                              5     0.5   Biaxial PET                                                                           1.62 890    15                                       Ex. 2                                                                         Comp.                                                                              3     1.0   Unstretched PE                                                                        1.51 743    16                                       Ex. 3                                                                         Comp.                                                                              3     0.5   Biaxial PP                                                                            1.61 525    16                                       Ex. 4                                                                         Standard                                                                           --    --    --      1.30 --     22                                       value                                                                         __________________________________________________________________________     *.sup.1 Biaxial: Biaxiallystretched, Uniaxial: Uniaxiallystretched.           PP: Polypropylene, PET: Polyethylene terephthalate, PE: Polyethylene.    

We claim:
 1. A wafer processing film comprising a base film, an adhesivelayer provided on one surface of said base film and a polypropylene filmarranged on said adhesive, layer,, the surface roughness of the surfaceof said polypropylene film in contact with said adhesive layer being notgreater than 2 μm.
 2. The wafer processing film according to claim 1,wherein the Shore "D" hardness of said polypropylene film is at least30.
 3. The wafer, processing film according to claim 1, wherein saidpolypropylene film is a biaxially stretched film.
 4. The waferprocessing film according to claim 1 or 3, wherein said polypropylenefilm contains a lubricant in a proportion not greater than 0.5 part byweight per 100 parts by weight of polypropylene.
 5. The wafer processingfilm according to claim 1 or 2, wherein said polypropylene film containsa lubricant in a proportion not greater than 0.1 part by weight per 100parts by weight of polypropylene.